Bring a lot of fire magic. Then use that fire magic to beat all the tentaclaws first then you go beat up the Leechgrave. Repeat that process if you can't beat the Leechgrave in 1 try. Also bring around 15-30 casts of fire magic.
Thanks! I have been stuck there for a couple days, because my friend kept telling me to freeze them, and it didn't work out very well. Finally!! Yay!! (I think there's a cutscene next, too. I love it when there are cutscenes.)
Your welcome.